On the island of Borneo, visitors can tour some of the biggest and longest cave passages in the world, all within a single cave system. The Mulu Caves beneath Mulu National Park were carved out of limestone over millions of years, and so far, 125 miles (200 kilometers) of underground cavern have been explored; it's possible there's three times that distance waiting to be discovered [source: FDSM].

The Mulu Caves boast several honors. According to the Sarawak Forestry Department, Mulu has the largest passage, in Deer Cave; the largest chamber, Sarawak Chamber; and the longest cave in Southeast Asia in the form of Clearwater Cave [source: FDSM]. The cave system offers tours for everyone from beginners to experienced spelunkers. Those looking for a real expert experience can attempt the 10 or so hours of total darkness required to navigate Sarawak Chamber.

And last on the list, for those afraid of the dark, a cave with some natural illumination.